In a rolling bearing such as to be used to support a machine tool spindle or the like in high speed rotation, a lubrication oil is jetted or sprayed into a space between an inner ring and an outer ring for lubrication of the inside of the bearing.
In the high speed rotation of the spindle, the lubrication oil should assuredly be supplied to the inside of the bearing against a centrifugal force and a high-speed air flow occurring adjacent an end face of the bearing due to rotation of rolling elements and a retainer. Therefore, a problem has been arisen with lubrication of the inside of the bearing, particularly, lubrication of a raceway groove of the inner ring which is prone to insufficient lubrication.
For the lubrication of the inside of the bearing, various under-race lubrication methods have been known, in which the lubrication oil is jetted toward the rolling elements from an oil supply channel having an orifice in the inner ring raceway groove formed on the inner ring.
One exemplary under-race lubrication method is disclosed in Japanese Unexamined Patent Publication No. 6-235425 (1994) which proposes an angular bearing apparatus, wherein a lubrication oil jetted from a nozzle or the like is retained in an annular space formed in an inner ring spacer and jetted toward rolling elements from the annular space through an oil supply channel formed in the inner ring spacer.
In the apparatus disclosed in this publication, an outlet port of the oil supply channel formed in the inner ring spacer is located as close to the rolling elements as possible. In this respect, the spacer has a relatively great axial size and the inner ring has a much smaller axial size than an outer ring, so that the inner ring spacer is inserted radially inwardly of the outer ring. However, this arrangement suffers from the following drawbacks (1) to (4):
(1) Since a mechanism defining the annular space is provided in a member separate from the inner ring, the bearing apparatus has a complicated construction and a greater size. PA0 (2) If there is a great axial size difference between the inner ring and the outer ring, it is difficult to control the axial size difference (so-called stand-out). In a conventional case where the stand-out corresponds to a pre-load, it is merely necessary to place the inner ring and the outer ring on a surface plate and to determine a height difference between the inner and outer rings with inner end faces of the inner and outer rings being defined as a reference surface. In the case of the apparatus disclosed in the aforesaid publication, however, it is necessary to even up the heights of the inner and outer rings by placing the inner and outer rings on a stepped jig placed on a surface plate and to determine a height difference between the inner and outer rings with the inner end faces of the inner and outer rings being defined as a reference surface or with outer end faces of the respective rings being defined as a reference surface. This approach is troublesome. PA0 (3) Since the inner ring having a smaller axial size suffers from greater expansive deformation due to a centrifugal force, the engagement between the spindle and the bearing apparatus is loosened, resulting in easy occurrence of creep and the like. Further, the inner ring having a smaller axial size is liable to be inclined with respect to the spindle, resulting in a tendency toward a greater degree of misalignment. PA0 (4) Since the inner ring spacer has a greater axial size and an end portion thereof adjacent to the inner ring has a greater wall thickness to be formed with an annular groove, the end portion of the inner ring spacer is particularly prone to expansive deformation due to a centrifugal force. The expansive deformation of the end portion of the inner ring spacer causes a change in the axial size of the inner ring spacer, resulting in variations in the pre-load.
One approach to these problems is disclosed, for example, in Japanese Utility Model Publication No. 6-37624 (1994), which proposes a rolling bearing unit which is adapted to simultaneously achieve the cooling and lubrication of a bearing and includes an annular groove formed in one end face of an inner ring for retaining therein a lubrication oil jetted from a nozzle, a plurality of through-holes extending through the bottom of the annular groove axially of the inner ring for cooling, and an oil supply channel for communication between a midportion of one of the through-holes and an orifice of the one through-hole located on the side of an inner ring raceway groove for lubrication.
In the rolling bearing unit disclosed in the Japanese Utility Model Publication No. 6-37624, however, the cooling through-holes each have a relatively large diameter to allow for passage of a great amount of the oil for cooling and, hence, have a smaller flow resistance. On the other hand, the oil supply channel branched from the midportion of the one cooling through-hole for the lubrication has a great flow resistance because the orifice thereof on the side of the inner ring raceway groove is narrowed due to presence of a rolling element. Therefore, virtually no lubrication oil flows into the oil supply channel.
Further, where the inner ring is formed with the annular groove, there is generally an area difference between the one end face of the inner ring formed with the annular groove and the other end face of the inner ring not formed with the annular groove. If the opposite end faces of the inner ring having different areas are to be simultaneously polished, the end face having a greater area (not formed with the annular groove) tends to be insufficiently polished due to an insufficient polishing allowance, so that the insufficiently polished end face needs to be subjected again to the polishing process. This leads to a reduction in working efficiency with an additional expense in time and labor. If the opposite end faces of the inner ring are to be separately polished, a similar problem arises without any additional means for applying different axial pressures to the inner ring at the polishing of the respective end faces.
In recent years, an oil mist method and an oil air method have often been utilized for the supply of the lubrication oil for the aforesaid under-race lubrication. The oil mist method is to spray the lubrication oil, while the oil air method is to jet a small amount of the lubrication oil together with air at predetermined time intervals.
In these supply methods, the lubrication can be achieved with the use of a relatively small amount of the lubrication oil, and the use of a smaller amount of the lubrication oil advantageously avoids exertion of an excess inertial load on the spindle. However, the amount of the lubrication oil to be supplied is significantly influenced by the flow of the air, so that stable lubrication is impossible.
More specifically, when a rolling element to be supplied with the lubrication oil passes over the orifice of the oil supply channel in the rolling bearing unit, virtually no space is present between the orifice and the rolling element. Therefore, the air hardly flows through the orifice, thereby failing to spray the oil mist or the oil air directly onto the rolling element. This results in insufficient lubrication.
In view of the foregoing, it is an object of the present invention to provide a rolling bearing unit which is capable of smoothly supplying a lubrication oil to rolling elements for the under-race lubrication employing the oil air method or the oil mist method.